Carrageenan is a complex mixture of sulphated polysaccharides comprising linear polymers of 1,3 bound .alpha.-D-galactose units and 1,4 bound .alpha.-D-galactose units with the following generalised structure: EQU .fwdarw.3.alpha.)A(1.fwdarw.4.beta.)B(1.fwdarw.3.alpha.)A(1.fwdarw.4.beta.) B(1.fwdarw.3.alpha.)A(1.fwdarw.4.beta.)B (1.fwdarw.
in which A and B represent galactose derivatives from two different groups. The molecular weight of useful commercial carrageenan is generally from about 500,000 to about 1,000,000. Polymers with a molecular weight below about 100,000 are not generally classified as carrageenan. Carrageenan is used extensively in the food industry as an emulsifier, a gelling agent and a thickening agent.
Carrageenan is normally soluble in warm water, in which it forms high viscosity solutions, and insoluble in most organic solvents. All types of carrageenans form complexes with proteins.
Various types of carrageenan designated as kappa, iota, lambda, ny and my carrageenan are known. The different types are differentiated according to the nature of their repeating galactose units. The most important carrageenan types for commercial purposes are kappa, iota and lambda carrageenan (Kirk-Othmer (ed): Encyclopedia of Chemical Technology, 3rd edition, 1980, p. 53).
Portions of the polymer chains in some types of carrageenan (kappa/iota) can form double helix structures and thus a 3-dimensional network which results in gel formation. Carrageenan gels are thermoreversible. The temperature at which the transition from gel to sol occurs (the gel's melting point) is between 40.degree. C. and 70.degree. C., depending upon the concentration and presence of cations.
In aqueous solution, the various types of carrageenan react differently towards different cations as follows:
kappa carrageenan: precipitates (gels) with K.sup.+, Ca.sup.++, Mg.sup.++, Ba.sup.++, Sr.sup.++ and NH.sub.4.sup.+, and is insoluble in solutions containing these ions. The strongest gelation is achieved with K.sup.+. No gelation occurs with Na.sup.+, and Na salts are soluble. PA1 lambda carrageenan: does not precipitate (gel) with the cations listed above. All salts are soluble. PA1 iota carrageenan: essentially like kappa carrageenan, but the strongest gelation is achieved with Ca.sup.++. PA1 (i) the treatment in the heated basic solution may result in a certain disintegration of the seaweed structure which inevitably leads to release of carrageenan whereby the solution becomes highly viscous and the yield of carrageenan in the final products is reduced, PA1 (ii) they are not generally suitable, especially not when seaweed species having a high content of lambda carrageenan is used as the starting material, since this type of carrageenan will be dissolved in the basic solution; the use of pure sodium-containing bases will lead to loss of kappa- and iota-carrageenan, since the sodium salts hereof are soluble in water, PA1 (iii) the resulting SRC products have a limited range of applications primarily due to coloured and odorous impurities and their high content of cellulose structures and other insoluble seaweed substances giving an undesired "cloudiness" in the products in which they are used. PA1 Carrageenan must be substantially insoluble or at the most only slightly soluble in the solvent, since the method is based upon a basic modification of the carrageenan as defined above taking place at its natural location in the seaweed, i.e. in situ. Any carrageenan that is dissolved by the solvent will tend to be lost to the reaction mixture, thereby decreasing the yield of carrageenan in the final product. A further disadvantage resulting from dissolved carrageenan is an increased viscosity of the reaction mixture which makes separation of the treated seaweed material difficult. PA1 The solvent should preferably be water-miscible, so as to enable a homogeneous mixture to be prepared and maintained without the use of, e.g., an emulsifier or excessive agitation. PA1 The solvent must allow the seaweed to be maintained in a structurally essentially intact condition. This is due to the fact that the basic modification of the carrageenan is dependent upon the seaweed being swollen and permeable, so as to allow passage of reagents (the base) into the seaweed as well as passage of dissolved matter (i.e. dissolved cellulose, colouring matter, protein, starch, etc.) out of the seaweed. However, since the carrageenan matrix functions as a "container" in which the modification reaction takes place, the structure of this matrix must remain intact and must not disintegrate, as disintegration results in the formation of a paste which is difficult to handle. PA1 a mixture of the solvent and water (135 g, containing the type and amount of solvent to be tested) is mixed in a 150 ml 3-necked flask and heated while stirring to the temperature at which base modification is to take place (a salt such as sodium chloride may be added to the mixture, depending on whether such a salt is to be present during the base modification). A conductivity electrode is placed at the surface of the mixture and the base to be used is added gradually while stirring slowly. When the conductivity changes abruptly, the point has been reached at which the reaction medium begins to separate into two phases, an aqueous bottom phase with a high base content and a top phase containing the majority of the solvent and having a low base concentration. The concentration of the solvent, water and the base is plotted in a phase diagram for the chosen salt content. If a given base concentration (enabling a single liquid phase to be maintained) is sufficient for the desired reaction, 15 g of chopped seaweed material is added while stirring slowly. PA1 a colour change from red to green for seaweed material of the family Gigartinaceae indicates that the base has penetrated into the seaweed material, and the amount of liquid that can be drained from the seaweed after heat treatment for a few minutes is a measure of the amount of liquid which has been absorbed. To check for possible incomplete swelling, samples of the seaweed can be taken and investigated under a microscope for red areas into which the base has not penetrated. To check for possible dissolution of the seaweed material, the drained off liquid phase is refrigerated; if carrageenan has been lost to the liquid due to dissolution of the seaweed, this will be shown by gelation in the cooled liquid.
These properties can be employed for selective extraction of kappa/iota and lambda carrageenan (see, e.g., Smith et al., Can. J. Chem. 33, 1352 (1955)).
Carrageenan contains galactose units which are sulphated in the 6-position. These can be converted into 3,6-anhydro galactose units (elimination of sulphate by ring formation) by treatment with a base. The resulting carrageenan product containing 3,6-anhydro galactose units shows improved gel properties.
The polymer chains in carrageenan can be broken by treatment with an acid (hydrolytic depolymerization) or by treatment with hydrogen peroxide (oxidative depolymerization). By the above-mentioned base treatment and a hydrolytic or oxidative depolymerization, carrageenan products having optimum gelling properties and viscosity for specific purposes may be obtained.
Carrageenan is found in seaweed of the class Rhodophyceae (red algae) from which it can be isolated. Carrageenan does not exist as a free polymer in the red algae, but constitutes a part of the "skeleton" of the algae.
The occurrence and distribution of the various carrageenan types in Rhodophyceae is dependent on, among other things, the species, location and life cycle of the seaweed. Carrageenan is found in species belonging to the families Gigartinaceae and Solieriaceae and particularly in the species belonging to the genera Gigartina, Chondrus, Eucheuma and Iridaea.
Red algae of the family Gigartinaceae, e.g., Chondrus crispus and Gigartina stellata, synthesize kappa and lambda carrageenan in different growth stages: kappa carrageenan in the male and female stage and lambda carrageenan in the asexual growth stage. The lambda/kappa ratio in isolated carrageenan from a species of algae is thus effected by the relative dominance of one or the other growth stage at the time the algae is "harvested" as well as by the location at which the algae grows. By use of vegetative propagation of algae from a given growth stage it is possible to obtain an algal material which is consistent with regard to content and distribution of carrageenan. Algae isolated from a given growth stage can be propagated vegetatively, thus maintaining this stage, thereby making it possible to obtain an algal material with a desired content of a given carrageenan type.
Red algae from the family Solieriaceae, e.g., Eucheuma cottonii and Eucheuma spinosum, synthesize essentially kappa and iota carrageenan, respectively.
The taxonomy of the seaweed genera and species, especially the genera Gigartina and Iridaea, is a matter of discussion. The Gigartina radula species are often identified as one or more Iridaea species. Furthermore, commercial designations differing from the botanical names according to present classification are often used, which causes identification problems. In the present context, the botanical names according to present classification are used.
The traditional process for the production of commercial carrageenan products comprises extraction of carrageenan from fresh or dried seaweed in hot water at a basic pH. The aqueous extract, which contains about 1% carrageenan, is filtered to remove insoluble material (cellulose, hemicellulose, etc.). The filtered extract, which optionally can be concentrated to about 4% and subjected to various purification treatments such as filtering with activated carbon, bleaching, etc., is then treated with an alcohol or with a salt to precipitate the carrageenan. Carrageenan prepared in this manner is generally referred to as "purified carrageenan" (PC).
The production of PC requires high energy consumptions and may involve substantial environmental pollution. Therefore, several attempts have been made to provide less costly carrageenan products. Such products, which are generally referred to as "semi-refined carrageenan" (SRC), are commercially available. Specific types of SRC are also known as "KOH-treated seaweed", "alkali-treated carrageenan", "Philippine Natural Grade" (PNG), and "Processed Eucheuma Seaweed" (PES). SRC is prepared by heat-treating whole seaweed in aqueous basic solutions under conditions which modify the carrageenan into 3,6-anhydro galactose units by removing the sulphate groups.
Examples of such SRC products are disclosed in JP 57.19942, which describes a method in which algae are heat treated in an aqueous solution of potassium carbonate and sodium hydroxide, following which the algal material is washed several times with water and potassium dihydrogen phosphate solution and finally dried and crushed to give the product which may, e.g., be applied in jams and pet food; in U.S. Pat. No. 4.443.486, which discloses a carrageenan stabilizing agent for use in milkbased products, prepared by alkali treatment of seaweed of the species Eucheuma cottonii; and in JP 53.107990, which discloses a method in which an algal material is treated with an aqueous potassium hydroxide solution at 70.degree.-95.degree. C., after which the treated material is washed with water and comminuted to obtain a product which may be used as a silkworm feed.
However, the known processes for the production of SRC products involves at least the following disadvantages: